1. Field of the Invention
This application relates to optical fibers including, for example, rare earth doped optical fibers and fibers with large effective area, which can be used for example in fiber lasers and amplifiers, as well as methods of making such fibers.
2. Description of Related Art
Fiber lasers have shown great promise over the last decade over their solid state counterpart due to a variety of advantages. Fiber lasers are easy to manufacture, are more efficient in heat dissipation, more stable, produce better beam quality, and are more reliable and compact.
Major limits in scaling up power in fiber lasers include nonlinear effects and optical damage, which are a direct consequence of tight confinement of the optical mode in the laser. There are a number of nonlinear effects in optical fibers. Self-phase modulation dominates in high peak power ultra short pulse generations. Raman scattering is one of the major limitations for longer pulses and CW operation. Brillouin scattering dominates in narrow spectral line width application.
There have been many studies on how to counteract these nonlinear effects. Some level of self-phase modulation can be balanced by dispersion in self-similaritons. Raman scattering can be reduced by W-type waveguide design to increase loss at Stoke wavelengths. Brillouin scattering can also be reduced by reducing acoustic waveguiding. Since all of these nonlinear effects are a direct consequence of high optical intensity in the optical fiber core, an increase of core size, which is equivalent to increase of effective mode area, can effectively reduce optical intensity and consequently nonlinear effects.
Multimode fibers with larger core size can be used to operate as near diffraction limited amplifiers in the presence of appropriate spatial filters and/or selective modal excitation of the fundamental mode. The use of multimode fibers enables the increase of core size beyond that offered by single mode fibers. See, e.g., Fermann et al in U.S. Pat. No. 5,818,630, which is incorporated by reference herein in its entirety. As fiber becomes more multi-mode with an increase of core size, easy launch and robust propagation of fundamental mode are also increasingly important in these fibers in order to maintain good beam quality.
Another effective method of reducing nonlinear effect is to use short length of fiber. This approach involves highly rare earth doped host glass.
What is needed therefore are glasses that provide for large core and/or high doping.